1. Field of the Invention
The present invention relates to an industrial sponge used for cleaning surfaces. More specifically, the invention is a synthetic industrial sponge made of polyvinyl acetal, and a method for extracting residue from the sponge to yield a sponge that leaves less particulate matter and has a longer life than conventional sponges, and a method of cleaning semiconductor material using the sponges.
2. Description of the Related Art
The use of synthetic sponges, made of polyvinyl acetal for example, is well known. For example, U.S. Pat. No. 4,566,911 discloses a scrubbing device using a polyvinyl acetal elastic material. Conventional sponges have been shaped in various forms, such as rollers and pads. These sponges are used to clean critical materials such as thin film disks, semiconductor wafers, data storage media, and other materials requiring removal of particles in the 0.1 micron range, i.e. “ultimate cleaning”. Conventional synthetic sponges have a polymer structure with “dead end pockets” formed therein that trap residue and trace amounts of metals. As the sponge wears, these metals can come out of the sponge in the form of particulate matter. Such particulate matter can damage the surfaces that are to be cleaned. Further, this type of sponge has tiny fibrils in the pores thereof that are a result of spaces between the pore forming chemical additive during a cross-linking reaction. “Cross-linking” is the formation of ester bonding between chains of the two adjacent hydroxyl groups that occur with the reaction of polyvinyl alcohol and aldehyde. This reaction hardens and strengthens the resulting material.
U.S. Pat. No. 4,098,728, the disclosure of which is incorporated herein by reference, discloses a polyvinyl acetal sponge and a method for making the same. In this method, pore spaces in the sponge are formed by gas bubbles; not pore forming chemical additives such as starch/sulfate combinations. Because the sponge disclosed in this patent does not have any starch residue, or the like, it has been particularly useful in medical applications in which starch residues can cause a foreign body reaction when in contact with mucosal tissue.
Also, conventional sponges have been used to form cleaning rollers or the like having projections formed on a surface thereof. However, the conventional projections have walls that extend perpendicular to an outer surface of the sponge body. Such a configuration creates stress concentration areas. Accordingly, the projections break off of the roller surface after continued use. Therefore, conventional sponges having projections have a short useful life.
Finally, extraction processes, also known as purification processes, are used to remove particulate matter and residue from the sponge before the sponge is used for cleaning. However, conventional extraction processes have not adequately removed residue to the extent necessary for reliably cleaning semiconductor wafers or similar materials.